Small Makefile improvements

- fix old build commands still using OFLAGS
- make everything work with --warn-undefined-variables, which can be useful
  for debugging typos
diff --git a/tests/Makefile b/tests/Makefile
index 73f9187..a33f8a7 100644
--- a/tests/Makefile
+++ b/tests/Makefile
@@ -37,6 +37,9 @@
 ifdef SHARED
 SHARED_SUFFIX=.$(DLEXT)
 endif
+else
+EXEXT=
+SHARED_SUFFIX=
 endif
 
 # Zlib shared library extensions: